Tanning



- Io Drawing.

Patented Aug. 6, 1929.

- UNITED STATES FRANK s. Low, or NIAGARA FALLS, NEW YORK.

TANNING.

This invention relates totanning; and it comprises a method of converting hide into leather wherein a tanning bath is produced by the solution of an anhydrous chromium chlorid .in water, this chlorid being sometimes chromous chlorid but more frequently chromic chlorid in the presence of some chromous' chlorid or of a reducing body adapted to form chromous chlorid, such as ferrochrome, untanned hides being treated by such a bath; all as more fully hereinafterset forth and as claimed.

Chromium has three states of oxidation, chromium trioxid,- ()rO (chromic acid); chromic oxid (sesquioxid) C50 and chromous oxid, CrO. As will be noted, the ratio of oxygen to chromium in the trioxid is twice as great as in chromic oxid; and that in chromous oxid is two-thirds that in chromic oxid. In recovering chromium from its ore, chrome iron stone, it is usual to roast the ore in the presence of air'and alkali, thereby oxidizing the contained Cr O to form a chromate with the alkali present, this chromate being afterwards leached out. From the chromate other chromium compounds are prepared, if wanted. In chrome tanning, the active agent is Or o, which is taken up from slightly basic solutions by hide with the production of leather; the expression Cr Q, being used for convenience and without theory as to whether it is actually 013,0 or some hydrated form ofthe oxid in solution or colloidal suspension or forms a component of a basic salt. Not much is actually known as to this. Chrome tanning can be effected by the use of a.bichromate, usually sodium bichromate. The CF03 of the chromate is converted into C130, by reduction. As a reducing agent, glucose is sometimes added, together with sulfuric acid to combine with the base i of the .chromate. In making chrometanning baths, sodium bichromate has the advantage of being a primary commercial material, other chromium comounds being usually made from it, and the urther advantage that it is a definite salt, not of a hygroscopic nature and is easily packaged, shipped and handled. But in the tan bath, it must undergo complex changes and the bath becomes loaded with inert products of reaction.

In tanning, whatever the theory of the action that occurs, a bath which will tan Application filed August 13, 1925. Serial No. 50,137.-

invariably contains a salt of chromium in the state of oxidation corresponding to chromic oxid; this salt containing less of the negative radicle ((31,30 etc.) than corresponds to the normal salts, CrX In other words, a basic salt is present; this salt being usually considered CrX OH in which X denotes a negative radicle.

For the present purposes, dissolved chromic salts, such as the sulfate and chlorid, may be considered as Cr,,() held in solution by the corresponding acid. From a salt containing the full theoretical amount of acid, namely 6HCl or 3H SO, for each (317 0 hide takes up but little C130,. In a way, tanning may be considered as a sort of competition between hide substance and acid for (lr o and the best tanning results are obtained from solutions in which the amount of acid is about two-thirds that theoretically required by the Or,(),; that is,

from a basic salt which may be regarded at I pleasure as Cr O,.4HCl or Cr Cl,,(OH) or CrCl .OH. The three formulas all mean the same thinga basic chromic salt. Such a salt is produced in the course of all tanning processes and as its basicity lessens with fixing of Cr by the hide, small amounts of soda ashare added to restore the basic condition.

It is one of the objects of the present in vention to enable the tanner to produce directly such a bath containing a basic chromium salt and capable of tanning from the start without the necessity of initially adding bases, acids, glucose, etc.

While the chromates have the advantage that they are relatively cheap, pure and easily obtainable, they present certain disadvantages in this relation. In use,there is a necessary reduction of chromium trioxid to sesquioxid before tanning can be done; and an adjustment of base-and-acid relations. For various reasons, chromium chlorid is a more desirable body for making the tanning baths,'but unfortunately it has not been possible to obtain it readlly in a salt.

I have found that excellent results can be obtained in using as a. material for tanning baths an anhydrous chromium chlorid .prepared in the dry 'way; this chromium chlorid being either CrCl or CrCl or a mixture of the two. Upon; exposure to chlorin at a proper temperature, ferrochrome is converted into solid chromium chlorid and ferric chlorid,the latter vaporizing. By this method, it is easy to prepare either chromous or chromic chlorid in a state of purity. Anhydrous chromic chlorid, free from chromous chlorid, is a purple or violet crystalline mass which is insoluble in water and is readily stored, shipped and handled.v It is not in any way corrosive to metals and there is no difficulty in packaging it. In theserespects it has the advantage over any commercial hydrated form of chlorid. .In the presence of a little chromous chlorid,,anhydrous chromic chlorid becomes soluble in water; the proportion of chromous chlorid necessary to efi'ect this being extraordinarily little. In making chromic chlorid, it is practicable to turn out a product containing this small amount. Such a product while having all the other properties and advantages of the anhydrous purple chlorid just described, can

chromic chlorid the tanner can readily make pure baths of definite composition which however require addition of a little alkali to produce a basic salt.

Chromous chlorid is a yellow micaceous material soluble in water. Its solutions in water readily'oxidize to the chromic form and give a basic chromic chlorid containing two-thirds the chlorin necessary for normal chromic chlorid in other words, giving solutions at once adapted for tanning. I

In the present invention, these anhydrous chromium chlorids may be utilized in various ways in making standardized definite tanning baths without the necessity and nuisance of "the use of glucose and sulfuric acid. The'anhydrouspurple chromic chlorid in its insoluble form can be shipped and stored, as stated, without trouble and.

can be readily brought into solution by the tanner by the use of a little reducing agent of any kind; something which will form chro- -mous chlorid. Zinc may be used. Incidentally, zinc as a reducing agent disposes of part of the chlorin and the solution formed is to that extent equivalent to an oxychlorid. The soluble form has practically the same advantages but requires no addition of reducing agent since it already contains some I (chromous chlorid).

' A very simple and efiicient way of making a tanning bath is to use a mixture of fine ground ferrochrome and anhydrous chromic in water. With -n'1ust be treated with alkali to obtain a basic chlorid. On treating this "mixture with hot itself is employed as a tanning bath. The

CrC-l quickly takes up oxygen and is converted into the equivalent of Cr Ol O, or 'a soluble basic chlorid.

Aside from their convenient physical nature, the preparations in question have the advantage of offering the tanner something which can be made of definite composition as regards chromium content and which is very rich in chromium. The Cr O equivalent of chromic chlorid is about the same as that of sodium bichromate, while that of chromous chlorid or of a mixture of chromic chlorid and ferrochrome is considerably higher.

In operating under the present invention, the preparation of tanning baths is exceedingly simple. Anhydrous chromous chlorid in the desired amount may be dissolved in hot water and the bath directly used. Basic chromic chlorid is formed as rapidly as hide will take up C130 Additions of soda are made from time to time after the first tanning action is complete. Or, in lieu of using chromous chlorid directl the anhydrous chromic chlorid may be aced in hot water with a little zinc or a ittle finely ground ferrochrome and the solution directly used as a tanning bath. For the convenience of the tanner, it is possible to furnish a fine ground preparation of anhydrous insoluble chromic chlorid and fine ground ferrochrome. To obtain the maximum amount of chromous chlorid, using commercial ferrochrome, a ratio of 6 parts anhydrous chromic chlorid to 1 part of ferrochrome is advisable. However, considerably smaller amounts of metallic ferrochrome will sufiice to render chromic chlorid soluble. The mixture on treatment with hot water goes into solution, giving a tanning bath. In this bath, using the full amount offerrochrome, the chro- 'mium is mainlycontained as chromous chlorid, which, however, quickly changes to the chromic form by oxidation, giving a solution of chromic oxychlorid. Oxidation may be hastened and facilitated by aerating the bath. In lieu of using ferrochrome, powdered zinc may be used, but there is no ad-.

vantage in this substitution. The preparations mentioned may be made, shipped and stored commercially and directly 'used by the tanner. On the addition of water, the described action takes place.

Tanning may be done in the usual ways, by drumming, etc. In drumming the agita tion of the bath provides aeration. v

What I claim is 1. In tanning hides and skin, the process which comprises exposing untanned hide to the action of a solution containing chromic and chromous chlorids.

2. In tanning hides and skins, the process which comprises exposing untanned hide to the action of a solution containing chromous chlorid.

3. In tanning hides and skins, the process which comprises forming a solution containing chromous chlorid and exposing untanned hide to the action of the resultant bath under conditions permitting oxidation of the chromous chlorid to basic chromic chlorid.

4. In tanning hides and skins, the process which comprises bringing anhydrous chromic chlorid into solution with the aid of a red ucing agent and tanning untanned hide with the resultant bath under conditions permitting oxidation of the chromous chlorid formed in the bath.

5. In tanning hides and skins, the process which comprises bringing anhydrous chromic chlorid into solution with the aid of ferrochrome and tanning untanned hide with the resultant bath under conditions permitting oxidation of the chromous chlorid formed in the bath.

6. In tanning hides and skins, the process which comprises exposing untanned hide to an aerated bat-h containing chromous chlorid.

7. In tanning hides to make leather the process which comprises submitting un-.

tanned hides to the action of a chromium chlorid solution containing chromium in the FRANK s. LOW.

l'iti 

